The Author making two Journies to Manchester, purposely to observe the Process by Mr. Sadler; found that his Balloon was inflated in two Hours each Time; by Means only of the two smaller _identical_ fermenting Vessels which Mr. Lunardi afterwards purchased; but the Levity procured by the former, tho’ he also expended 16 Bottles, was by no Means so great as that gained with the Assistance of the great Vessel. It has likewise been remarked by the Author, who has made several Experiments to this End, that the Vessels always continued in Fermentation and Ebullition, with a _quick Pulsation_, for at least 24, and commonly during 48 Hours, after the Inflation was completed. And, that not more than the Depth of _half an Inch_ of Filings had been _calcined_ during the Operation: the rest being perfectly _bright_, and untouched by the Acid. _Observation. 2._ If therefore one Inch in Depth of Filings, be spread over the Bottom of each of the _smaller_ Vessels only; the proper Quantity of Water poured in; and _not more_ than two Bottles of Acid used at once, in each Vessel; also, as soon as the Fermentation begins to decline; other two Bottles, and a proportionable Supply of Water be added; if suffered to work double, triple, or quadruple the Time;—the Inflation will be as great, if not greater, for Instance, in six Hours with eight Bottles, and two small _Tubs_, as it woud in three Hours, with 16 Bottles, in the _same Vessels_. The small conducting Tin Tubes ought instead of four and a half, to be nine Inches Diameter: by which Means there will be no violent Pressure of Gass to endanger the Bursting of the Vessels: particularly if the Gass is not suffered to descend; but, on the contrary, according to Instructions already given, either to rise, or move, in an horizontal Direction, past the Evàcuatory, into the Balloon. 346. The Workmen may begin the Operation at twelve at Night, or at six in the Morning: and the Time previously fixed for the Exhibition, may be eight or ten Hours after the Operation has commenced. The Necessity of a Current of fresh Water, throu’ a Pipe of at least half Inch Bore, the larger the better, to supply the overflowing Cistern, cannot be too much _insisted_ on: as the Levity of the Gass almost wholly depends upon so trivial a Circumstance, as that of having a plentiful Supply of _cold fresh_ and _soft_ Water. 347. _Observation 3._ Supposing the Balloon +air-tight+, near half the Expence is thus saved in the Inflation. Besides the greater Probability of +calm+ Weather for the Inflation, if completed before X. in the Morning, more Time is given to remedy Accidents, and rectify Mistakes: the Warmth of the Air likewise encreases. But above all; if an upper Current carry the Balloon to Sea, the Aironaut may, (as before mentioned) drop into the Sea-Breeze, which will waft him safe back till IV. in the Afternoon, or even later. CHAPTER LXIX. MENSURATION OF HEIGHTS.